An analog buffer circuit is given as one of applications of an amplifier circuit. One of characteristics of the analog buffer circuit can be exemplified by having a linear relation between amplitude of an input signal and amplitude of an output signal. Some analog buffer circuits, in which the amplitude of the output signal has a relation of an amplification degree “1” with respect to the amplitude of the input signal, are also utilized. In this case, the output signal becomes a signal into which the input signal is reproduced as it is. This type of amplifier circuit is used for the purpose of isolating the input side and the output side so that the input side and the output side do not affect each other. In this case, the amplifier circuit drives a heavy load and forms a high input impedance on the input side.
Circuits in, e.g., FIGS. 1 through 3 are known as amplifier circuits that provide this type of function. FIG. 1 illustrates an example of a voltage follower using an operational amplifier. In the circuit in FIG. 1, in an idealistic state where a degree-of-amplification K is infinity and an input impedance is infinity, an output-to-input relation becomes Vout=Vin, and the output signal becomes a signal in which the input signal is reproduced with fidelity.
FIG. 2 illustrates an example of a source follower. The source follower is a circuit in which a drain is earthed, and a load is provided on the source side. In the circuit in FIG. 2, let Vgs be a voltage between the gate and the source, Vth be a threshold voltage of a transistor M1, β be an amplification factor of an electric current, ids be the current flowing to the drain from the source, Vi be an input voltage inputted to the gate, Vo be an output voltage output from a terminal on the source side and lout be an output current, and there are relations given in the following mathematical expressions 1-3.ids=β(Vgs−Vth)2   (Mathematical Expression 1)Vo=Vi−Vgs=Vi−Vth−(ids/β)1/2   (Mathematical Expression 2)ids=Ib−Iout   (Mathematical Expression 3)
FIG. 3 illustrates an example of a linear amplifier buffer. The linear amplifier buffer converts input voltages Vin+ and Vin− into the currents by use of a voltage/current converter. Then, the linear amplifier buffer linearly converts the thus-converted currents into the voltages by use of a resistance. In this case, a relation (which is termed a transfer function) between the input voltage Vin(=Vin+−Vin−) and the output voltage Vo(=Vout+−Vout−) can be expressed in the following mathematical expression 4. Herein, gm is a transconductance (mutual conductance) of the transistor. Further, if the transconductance gm is sufficiently large, an item of gm can be ignored with the denominator in the mathematical expression 4. As a result, it follows that the relation between the input voltage Vin and the output voltage Vo is determined by a resistance ratio Rload/Rd as in the mathematical expression 5.Vo=Vin×Rload/(Rd+2/gm)  (Mathematical Expression 4)Vo=Vin×Rload/Rd  (Mathematical Expression 5)
Note that a load capacity inserted in parallel with the resistance Rload and a parasitic capacitance parasitic to the output side are combined into Cload, at which time the relation between the input voltage Vin and the output voltage Vo can be expressed in the following mathematical expression 6. Herein, s is a complex variable.Vo=Vin×(Rload/Rd)/(1+s×Rload×Cload)  (Mathematical Expression 6)
[Patent document 1] Japanese Patent Laid-Open Publication No. 2007-43654